This invention relates to a tone signal processing device utilizing a digital filter and, more particularly, to a device of this type used in an electronic musical instrument or other instrument having a tone generation function or a digital voice processing device. Further, this invention relates to a tone signal processing device used in an electronic musical instrument of a type which generates a digital tone signal in plural channels on a time shared basis and, more particularly, to a device of this type controlling a generated digital tone signal with a digital filter and resampling it in synchronization with the pitch of the tone.
Use of a digital filter in a tone color circuit in an electronic musical instrument is disclosed, for example, in Japanese Preliminary Patent Publication No. 59-44096. The prior art digital filter carries out a filter operation with a regular sampling period which is determined depending upon the system in which the digital filter is used and filter characteristic obtained thereby is a fixed formant.
If a filter characteristic of a moving formant is to be realized in the tone color circuit using such digital filter, filter coefficient must be changed in accordance with the pitch of a tone signal applied to the circuit. This requires a large number of filter coefficients with a result that filter coefficient memory means of a large capacity is required and hence the device becomes of a large and complicated construction.
Further, in the prior art tone color circuit using the digital filter, means as shown in FIG. 33 for example is adopted as another means for realizing the moving formant filter characteristic. In this device, digital filters DF1-DFn realizing mutually different fixed formant characteristics for a plurality of tone pitches are provided in parallel, a digital tone signal is applied to a distributor DSTRB and the tone signal is distributed to one of the digital filters DF1-DFn in accordance with the pitch of the applied tone signal. The characteristic of each of the digital filters DF1-DFn is a fixed formant characteristic which is different depending upon the corresponding pitch so that these digital filters DF1-Dfn are used selectively in accordance with the pitch of the tone to be generated and filtering of a moving formant characteristic can be realized in effect by combining these digital filters DF1-DFn. This construction, however, requires a large number of digital filters so that this device also requires a large and complicated construction.
In an electronic musical instrument generating a tone signal in a digital fashion, the sampling frequency is not necessarily harmonized with the pitch of the tone and this gives rise to a problem of aliasing noise. For eliminating the problem of aliasing noise, a pitch synchronization technique is employed in which the sampling frequency is harmonized with the pitch of the tone. As an example of such prior art using the pitch synchronization technique, it is practiced to resample, with a sampling period which is synchronized with the pitch, a digital tone signal generated with a sampling period which is not synchronized with the pitch (U.S. Pat. No. 4,377,960).
On the other hand, to employ a digital filter in a tone color circuit of an electronic musical instrument is disclosed in, for example, the above mentioned Preliminary Patent Publication No. 59-4409. In employing a digital filter in a tone color circuit, however, it has not been conceived how the pitch synchronization should be realized.
If the prior art digital filter is simply applied to an electronic musical instrument of a pitch synchronizing type, a device realized will be one as shown in FIG. 34. In this device, digital tone signals of plural channels (n) generated on a time shared basis from a tone generation circuit 140 are latched by first latch circuits 1411-141n provided for the respective channels in response to timings signals CH1-CHn corresponding to the respective channels whereby the tone signals are released from the time division multiplexed state. Then outputs of the first latch circuits 1411-141n are latched by second latch circuits 1421-142n in response to pitch synchronizing pulses PSP1-PSPn synchronized with pitches of tones assigned to the respective channels whereby resampling synchronized with the pitches of the tones is performed. The digital filters DF1-DFn are provided in parallel for the respective channels so as to perform filtering channel by channel independently from one another and digital tone signals in a pitch synchronized state provided by the second latch circuits 1421-142n are respectively applied to these digital filters DF1-DFn. The operation speed of each circuit in such device will now be considered taking an example. Assume, for example, that the sampling frequency of a tone signal in the tone generation circuit 140 is a fixed rate in the order of 50 kHz. Since resolution of timing of generation of the pitch synchronizing pulses PSP1-PSPn is common multiple of the sampling frequency 50 kHz and the pitch of a tone, it becomes for example a high rate in the order of 400 kHz. Accordingly, the operation rate of the digital filters DF1-DFn must be one which is matched with the resolution 400 kHz of the sampling rate of the second latch circuits 1421-142n. If operation of respective filter orders for the digital filters DF1-DFn is to be performed in these digital filters DF1-DFn, the filter operation must be performed with an even higher rate which is 400 kHz multiplied by the order.
In the conventional digital filter, if a filter order is fixed to a predetermined order for reasons of circuit design, the order of the filter circuit is fixed to this order in terms of hardware construction. For this reason, there has been the problem that a filter characteristic (i.e., amplitude-frequency characteristic) realizable is limited depending upon the order fixed in terms of hardware construction. For example, frequency response characteristic of a filter of an odd number order having an impulse response shown in FIG. 6 is as shown in FIG. 8 whereas frequency response characteristic of a filter of an even number order having an impulse response shown in FIG. 7 is as shown in FIG. 9. When the order N is an odd number, level at .omega.=.pi. (where .pi. corresponds to 1/2 of sampling frequency fs) is not fixed to 0 but can be set at any desired value as shown in FIG. 8. When N is an even number, the level at .omega.=.pi. becomes always 0. As will be apparent from this, when the order N is an odd number, a high-pass filter characteristic can be realized by establishing a filter coefficient suitably but when the order N is an even number, it is difficult to realize a high-pass filter characteristic. Thus, the prior art device has the problem that there is a filter characteristic which it is impossible or difficult to realize with a order fixed in terms of hardware construction. For overcoming this problem, it is conceivable to provide plural filters of different characteristics in parallel or in series but this gives rise to another problem that hardware construction becomes enlarged.
Further, in the conventional digital filter, filter coefficients must be prepared individually in correspondence to all orders (i.e., in correspondence to all orders from 0-th to N-1-th in the case of a filter of N-th orders). This causes the problem that a filer coefficient supply device (e.g., a coefficient memory) becomes large. Besides, in designing a desired filter characteristic, values of filter coefficients of all orders must be considered and this involves a troublesome calculation. Particularly in a filter for a tone signal, the filter characteristic should preferably be established at a linear phase characteristic (i.e., phases of input and output waveshapes corresponding in complete linear characteristic), for such linear characteristic is not likely to produce distortion in the output waveshape.
Further, in the conventional digital filter, supply of filter parameters is performed in a single channel. For example, sets of parameters corresponding to various tone colors are stored in a filter parameter memory and a set of parameters corresponding to a selected tone color are read out and supplied to the filter. In this case, timewise change of the tone color can be effected by timewise changing parameters. Since, however, values of one set of parameters must be changed continuously, plural sets of parameters must be prepared in correspondence to one selectable tone color. Since the memory capacity of a memory is limited, the number of tone colors for which parameters can be stored is limited. Moreover, if parameters corresponding to tone colors which do not undergo timewise change and tone colors which undergo timewise change are to be stored together in a memory of a single channel, readout control must be made separately for these two types of tone colors which involves a troublesome operation. Besides, since the number of sets of parameters corresponding to the tone color is different one tone color from another, distribution of the number of addresses is troublesome and there is also likelihood that some addresses are wasted without being used.
Further, in a prior art tone color circuit of an electronic musical instrument using a digital filter, a set of filter parameters are supplied to digital filter and a filter characteristic (amplitude-frequency characteristic) is established in accordance with the supplied filter parameters. Filter parameters of plural sets are prestored in a memory in accordance with contents of tone color determining factors and a set of filter parameters are read out in accordance with contents of selected tone color determining factors.
In the prior art tone color circuit, if different tone color control is to be performed depending upon plural tone color determining factors (e.g., key touch, tone range, constant tone color selection information, information according to lapse of time, an amount of operation of a manual operator such as a brilliance operator etc.), plural sets of filter parameters must be stored in the memory with one-to-one correspondence to respective combinations of tone color determining factors. For example, in a case where filter parameters are stored individually in one-to-one correspondence to all combinations (22528 combinations) of forty-four tone ranges, sixteen key touch groups and thirty-two kinds of constant tone colors, the parameter memory is required to have a large capacity capable of storing 22528 sets of parameters.